1. Field of the Invention
This invention relates generally to nondestructive testing, and more particularly, to nondestructive magnetic particle inspection of ferromagnetic material and the apparatus therefor.
2. Description of the Prior Art
The use of electric current to create or induce a magnetic field in ferromagnetic material to test the material for flaws, defects or discontinuities, hereinafter called "defects", in areas of interest, such as weldments, is well known. Moreoever, the magnetic inspection method is well known for detecting both surface and subsurface defects which create detectable perturbations or distortions in the induced magnetic field. In general, the defects distort the induced magnetic field in the ferromagnetic test material producing a leakage field external to the test material and also creating opposing magnetic polarities across the defects which, among other things, will attract an accumulation of magnetic particles applied to the surface of the material if the magnetic field perturbation is of sufficient strength to penetrate the surface of the material.
Accordingly, magnetic particle inspection, as it is commonly known in the art, comprises three basic operations: creating or inducing a suitable magnetic field in the test material; applying a powder or suspension of magnetic particles on the surface of the test material in the area of the induced magnetic field and inspecting the material's surface for accumulations of particles, that is, indications of defects, e.g. cracks, holes, slag entrapment, lack of penetration of fusion due to an improper weld, and the like.
Although the magnetic field of the magnetic particle inspection method may be induced in the test material by a permanent magnet, the most commonly created magnetic fields for this type of nondestructive testing are induced by electric current flow, such as, direct, alternating and half wave rectified current. Direct current, for instance, induces a time invariant magnetic field in response to the constant direct current which penetrates deep within the material and, accordingly, affords subsurface defect detection. Alternating current, however, induces an alternating magnetic field, in response to the current, which is limited by skin effect to the surface of the material and, therefore, to surface defect detection only. Half wave rectified current, that is rectified alternating current, in effect is a combination of direct current and various harmonics of the alternating currents and affords surface and subsurface defect detection.
However, each of these magnetic inspection techniques are limited to a certain extent. For example, a direct current supply such as a storage battery is limited in the magnitude and duration of available current. Furthermore, the constant direct current induces a constant magnetic field which attracts the magnetic particles and holds them fixed in position along magnetic lines of force. And, in addition, the direct current test may cause material burn due to excessive continuous current. Alternating current, moreover, is limited to surface detection only, because the magnetic field strength associated therewith decreases exponentially into the material. However, alternating current provides a dynamic motion to the particles in response to the induced varying magnetic field. Half wave rectified current likewise produces a motion in the particle powder but the subsurface component of the induced magnetic field is limited to some extent by the rectified alternating current fields at the surface of the material, which may interfere with the subsurface defect detection.
Accordingly, for magnetic particle inspections of test materials in which half wave rectified current does not produce satisfactory results two tests are required, a subsurface or direct current test and a near surface alternating current test. Clearly, from an economic point of view a substantial savings can result from a single test system or one which readily performs both deep subsurface and surface detection. In addition, the prior art methods generally require the skill of a trained inspector to determine the nature, location and extent of the indications by examination of the influenced magnetic particle powder.
Therefore, there is a need to provide industry with an apparatus for nondestructive magnetic inspection of areas of interest of test materials which economically and efficiently inspects the material for both subsurface and surface defects, enhances the detection of deep subsurface defects and which improves the response of the particles to the magnetic field for ease of observation of any defects.